Knife Coating (floating knife) or Direct Coating
In Knife Coating, as seen in Figure 3, the liquid coating is applied to the fabric while being run at tension under a floating knife blade, the distance between the fabric and the knife blade determines the thickness of the coating. The blade can be angled and have different profiles to affect the coverage. For this process to be effective the liquid coating must be quite viscous in order to prevent it soaking through the fabric, the coating is then dried or cured.
This technique is best used for Filament? yarns as the staple fibres in spun yarns can protrude on the surface creating an un-even finish, but this is dependant upon the thickness of the applied coating. For this type of coating to be most successful the weave structure has to be quite tight and the fabric capable of being held taught.
Figure 3. Fung, W, 2002, Coated and Laminated Textiles, UK;Woodhead Publishing; http://books.google.co.uk/books/about/Coated_and_Laminated_Textiles.html?id=U0Rh-TrvMz4C
Direct Roll Coating
In this process coating liquid is rolled onto the fabric by a roller suspended in the coating solution, often a blade is positioned close to the roller to ensure not too much coating solution is applied.
Figure 4. Direct Roll Coating from;Sen, A K, 2008, Coated Textiles; Principles and Applications, 2nd Edition, USA;Taylor and Francis.
Also referred to as Padding?, this technique, widely regarded as a textile finishing technique, can in fact be used to add a variety of coatings, but this usually refers to a fibre coating for the application of micro or nano materials or chemical compositions.
As shown in figure 5, the fabric is submerged in the coating solution then the excess squeezed out in the rollers, which dictates the pick-up percentage, the fabric is then dried and cured.
Figure 5. Image adapted from; http://www.kenencoregroup.com/aloevera-finish.html
Calender finishing involves the fabric passing through a set of heated rollers to singe off any surface fibres and add lustre and smoothness. Calender coating is the same principle in which the fabric passes through heated rollers, but through this process a coating is applied as demonstrated in figure 6. This image demonstrates the simultaneous coating of both sides of the fabric with the thickness of the coating determined by the width of the nip in-between the rollers, more rollers used can provide a thinner coating.
Figure 6. Depicts calender coating. Image from; TPO coated PP fabrics and their applications http://www.fibre2fashion.com/industry-article/pdffiles/16/1526.pdf
Hot melt extrusion coating
Hot melt extrusion coating is applied in the same process as calendering with the coating being melted from granules fed to heated rollers which then nip the coating to the fabric. It is used to produce un-supported films and these freshly produced films are added direct to fabric. Its uses are mainly for Thermoplastic polymers such as Polyurethane, Polyolefin’s and PVC.
Foam finishing was developed as a more environmentally friendly version of the pad-dry-cure system, as the chemical applied requires less product in weight, but equates to a high surface area. Foam also ensures less wetting takes place, which requires less drying; furthermore waste is reduced in terms of residual liquor. This technique is useful in coating heavy fabrics such as carpets and can be used to effectively coat only one side.